The present invention concerns two-cycle engine, and more particularly to improvements made to loop scavenging type two-cycle engines characterized by the method of scavenging.
Among the prior art loop scavenging type engines, Schnurle type two-cycle engine is well known. The engine charges the air-fuel mixture into a crankcase from a carburetor via an inlet port, and then into a cylinder from a scavenging charge port thereof through a passage provided between the crankcase and the cylinder wall after applying compression force by lowering the top piston in the crankcase. The scavenging charge port for charging the air-fuel mixture opens toward the wall opposite an exhaust port inside the cylinder. Thus, the air-fuel mixture charged into the cylinder from the scavenging charge port climbs up in loops along the wall opposite the exhaust port, is guided into the cylinder head, compressed by rising of the piston and ignited. When combustion of the air-fuel mixture brings the piston down, the exhaust gas is discharged out of the cylinder through the exhaust port and new air-fuel mixture is charged into the cylinder from the scavenging charge port so that a portion of the exhaust gas remaining inside the cylinder is efficiently and completely discharged outside the cylinder from the exhaust port.
The Schnurle type two-cycle engine is advantageous in that the air-fuel mixture charged into the cylinder from the charge port climbs up in loops toward the cylinder head along the wall facing the exhaust port, and conducts scavenging in an efficient manner. However, in spite of the above mentioned advantages, this type engine is defective in that the air-fuel mixture still in the liquid state directly blows out from the exhaust port without burning immediately after it is charged into the cylinder from the charge port, and causes high hydrocarbon (HC) emission, harming the environment.
Such problems appear probably due to the following reasons. With this type engine, efficient scavenging is attempted by letting flow in loops the air-fuel mixture charged into the cylinder from the charge port toward the cylinder head along the wall opposite the exhaust port. In order for the air-fuel mixture to smoothly flow toward the wall opposite the exhaust port, the piston top surface should be smooth and free of irregularities. If the top surface is smooth, the air-fuel mixture from the charge port is discharged toward the direction the charge port faces, and becomes dispersed immediately after it is discharged in the cylinder, creating a layer which flows over a long distance in loops along the inner wall surface of the cylinder and another layer inside the first layer which flows toward the center of the cylinder. The second layer leaks outside from the exhaust port without burning in the liquid state.
Japanese Utility Model Kokai No. Sho59-28628 discloses a system to overcome the problem of a portion of the air-fuel mixture leaking from the cylinder through the exhaust port in the liquid state. According to this system, the air-fuel mixture suctioned into the cylinder from the scavenging charge port is not allowed to flow in loops toward the cylinder head along the cylinder inner wall but is directed toward the cylinder head directly through the center of the cylinder from immediately above the piston.
This system provides a guide passage for the air-fuel mixture at the top surface of the piston connecting the side area and the top surface of the piston, passes the air-fuel mixture from the scavenging charge port opened on the inner wall of the cylinder through the guide passage and directly sends the mixture toward the cylinder head, which is positioned above the piston top surface, from the outlet port of the said guide passage acting substantially as a scavenging port.
The system may be suitable for efficiently guiding the air-fuel mixture toward the cylinder head through the cylinder center since the mixture passes from the outlet port of the scavenger guiding passage, said outlet port acting as an scavenging port and opening over the top surface of the piston. The system, however, entails the problem in that the flow of the air-fuel mixture passing through the cylinder center alone cannot secure scavenging all the exhaust gas generated after combustion of the air-fuel mixture, leaving a portion of the exhaust gas inside the cylinder. The system is further defective in that the area of the piston top surface increases because of a hole bore from the side to the top of the piston as a guiding passage for the scavenging air, and that the volume of the combustion chamber formed between the piston head and the cylinder head becomes larger, thereby reducing the compression ratio inside the cylinder and deteriorating the combustion efficiency.
The above mentioned Japanese UM Kokai Sho59-28628 also discloses a construction for a scavenging system wherein a depression or a groove shaped like the letter V when viewed from above is provided on the periphery of the piston top surface as a means for efficiently flowing in loops the air-fuel mixture charged into the cylinder from the scavenger charger port, and the air-fuel mixture charged into the cylinder from the scavenging port is guided by the V-shaped groove on the periphery of the piston top surface toward the wall opposite the exhaust port.
Since this system also forcibly guides the air-fuel mixture charged into the cylinder from the scavenger port toward the wall opposite the exhaust port by the V-shaped groove, it is defective in that the volume of the combustion chamber formed between the piston head and the cylinder head increases by the additional volume of the V-shaped groove provided on the periphery of the piston top surface, the compression ratio inside the cylinder does not become high, and the combustion efficiency is poor.